PRECISE AND ACCURATE DETERMINATION OF SILICON ISOTOPE RATIOS BY MULTIPLE COLLECTOR ICP-MS

Silicon isotope ratios in rocks show variations of some d³⁰Si units of over 5‰, and it is likely that weathering processes and soil formation processes give rise to significant degrees of Si isotopic fractionation. Conventionally Si isotope ratios have been measured to a limited degree of success by fluorination of silicate rocks followed by analysis using a gas source mass spectrometry. The advent of multiple-collector ICP-MS offers a new technique with enhanced capabilities over conventional instrumentation. Measurement of silicon isotope ratios by multiple collector ICP-MS is plagued by the isobaric interferences of ¹⁴N₂ on ²⁸Si and ¹⁴N¹⁶O on ³⁰Si. However, using collision cell technology on the Axiom Plus multiple collector ICP- MS, these isobars have been significantly reduced to enable accurate Si isotope ratios to be measured. We have investigated the use of H, He and Ne collision gases to reduce these isobaric interferences and these results will be presented. In addition, we will show how data on the Axiom compare for the same samples with that have been measured by gas source isotope ratio mass spectrometry and discuss the implications from this data set.